It can be appreciated that Piezo element drivers have been in use for years. Typically, Piezo element drivers are comprised of transconductive mechanisms that develop mechanical force by utilizing the principles of either electrostriction or magnetostriction. These transconductive mechanisms can be utilized to perform highly efficient mechanical actuation as they return most of the energy transferred into the device. Energy losses are composed primarily of the actual energy required to produce the mechanical work. There are many power circuits known within the art that can be used to drive transconductive mechanisms. These include linear driver circuits that transfer the return energy from the transconductive mechanism to a non-regenerative load. Other driver circuits include regenerative capabilities that store the reverse energy from a transconductive mechanism into either a bypass supply capacitor, or into the symmetrically opposite half of the transconductive mechanism itself. Such circuits have been described in prior art related to the application of driving bimorph actuators. In such circuits, the load itself is assumed to be balanced and symmetric so that it can be used as an energy storage element in the energy balance of the overall system.
The main problem with conventional Piezo element drivers is that they are designed to drive only one side of a transconductive system. Such systems, when applied to a symmetrically coupled system such as a bimorph bender, produce large peaks in the power supply ripple current and are known to be inherently unstable. Circuits have been developed to specifically drive a symmetrically coupled system. These circuits, however, require the use of high side drivers and auxiliary high voltage power supplies. Another problem with conventional Piezo element drivers is that they require the use of a high voltage power supply to transfer energy to the transconductive system. The requirement for such a high voltage supply (typically in the range of 60V to several hundred volts) adds cost, complexity, and reliability issues to the overall system.
Existing products operate the transconductive system at a fixed power supply voltage and, in the case of a symmetrically coupled system such as a bimorph actuator, the total voltage (V) and thus the charge Q (where Q=CV) across the system capacitance (C) always remains constant. The sum of the voltages across each half of the bimorph is therefore always equal to V. Such an approach is limited to providing efficient energy regeneration in systems which only operate the bimorph actuator in a continuously alternating mode of deflection. Specifically, such an approach would fully charge one half of the bimorph, and then transfer all of that charge to the other half of the bimorph, resulting in a complete oscillation of the bimorph actuator between its full positive and full negative deflection state. The disadvantage of this approach is that efficient energy recovery cannot be accomplished if the bimorph actuator is to be operated independently in one direction or the other. For example, if the bimorph actuator is to be deflected in the positive direction, held there, and then returned to zero deflection in its equilibrated charge state, ½ of the energy stored in the transconductive element when at full deflection will have been lost as it is returned to zero deflection. This follows from the theoretical analysis that the energy (E) stored in such a transconductive system varies in proportion to the square of the voltage (E =½ CV2) across its capacitance. If such a system were to have all of its energy stored in only one of the symmetric capacitive elements of the bimorph actuator, then the total energy stored in that element would be equal to ETOT=½ CVmax2, where Vmax is the maximum voltage of the power supply. If the bimorph actuator is to be returned to its equilibrated charge state, the charge from one half of the bimorph is transferred to the other half of the bimorph actuator until both halves are of equal charge, resulting in equal voltages of Vmax/2 across each of the capacitive elements. The total energy stored in this equilibrated state is equal to ETOT=2*½*C*(Vmax/2)2, or ETOT=¼ CV2. Hence, only one half of the energy stored in the fully positive deflection state of the bimorph actuator is actually transferred to the equilibrated state, while the remaining energy must be temporarily stored by some other means, or be dissipated as losses. Deflecting the bimorph actuator to either its fully positive or fully negative deflected state would require the addition of ¼ CV2 of energy from the power supply or said alternate temporary means of energy storage. Such means of energy transfer can be greatly improved by not restricting the voltage across the bimorph to the power supply voltage, thereby allowing for a complete transfer of energy between its fully positive, equilibrated, and fully negative deflected states.
Another problem with conventional Piezo element drivers is that they utilize the approach of a fixed frequency of switching of the energy between the symmetric loads (in hundreds of kilohertz) and modulate the duty cycle (percentage of ON time) at that switching frequency to control the balance of energy in the system. This approach results in higher driver circuit switching loss than that created using a lower switching frequency or ‘Switch As Required’ (SAR) approach.
While these devices may be suitable for the particular purpose to which they address, they are not as suitable for providing an energy efficient switching topology that allows measured amounts of energy to be transferred to the inductive element from either a capacitive load or a low voltage supply.
The general purpose of the present invention, which will be described subsequently in greater detail, is to provide a new low voltage, low loss driver for capacitive load systems that has many of the advantages of the Piezo element driver systems mentioned heretofore and many novel features that result in a new low voltage low loss driver for capacitive load systems which are not anticipated, rendered obvious, suggested, or even implied by any of the prior art, either alone or in any combination thereof.
A primary object of the present invention is to provide a low voltage low loss driver for capacitive load systems that will overcome the shortcomings of the prior art devices.
An object of one aspect of the present invention is to provide a switching topology that allows measured amounts of energy to be transferred to an inductive element from either a capacitive load or a low voltage supply through the use of low voltage control signals, and will then automatically deliver that energy to an appropriate capacitive load.
An object of another aspect of the invention is to provide a low loss energy transfer circuit to transfer energy from one high voltage piezoelectric or capacitive load to another high voltage piezoelectric or capacitive load, with a common electrical center point, without the need for high voltage driver circuits to drive the switching elements that control the transfer.
An object of a further aspect of the invention is to provide a low loss means to increase the charge on either of the piezoelectric or capacitive loads to an arbitrary value within their rated maximum range without the use of a high voltage power supply.
Another object of one embodiment of the invention is to meet the preceding two objectives using a single inductive element to mediate the transfer of energy between the two piezoelectric or capacitive loads and to mediate the transfer of energy from a dual polarity low voltage supply to either of the two piezoelectric or capacitive loads.
An object of another aspect of the invention is to provide feedback signals to allow the precise control of energy transfer under dynamic operating conditions.
Another object is to allow the control of a bimorph piezoelectric actuator, a monomorph piezoelectric actuator and capacitor combination, a dual monomorph piezoelectric actuator configuration as the capacitive loads in the circuit, or a multimorph piezoelectric actuator with low losses during movement of the piezoactuator(s) and energy recovery through the energy transfer mechanism of the circuit.
Other objects and advantages of the present invention will become obvious to the reader, and it is intended that these objects and advantages are within the scope of the present invention.
To the accomplishment of the above and related objects, this invention may be embodied in the form illustrated in the accompanying drawings, attention being called to the fact, however, that the drawings are illustrative only, and that changes may be made in the specific construction illustrated.